linux-kernel - Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional into test metadata

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Message-ID: <aEsUGP8xPTDjG0ob@nvidia.com>
Date: Thu, 12 Jun 2025 10:53:28 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	Shuah Khan <shuah@...nel.org>, Shuah Khan <skhan@...uxfoundation.org>, "Willy
 Tarreau" <w@....eu>, Thomas Weißschuh
	<linux@...ssschuh.net>, Kees Cook <kees@...nel.org>, Andy Lutomirski
	<luto@...capital.net>, Will Drewry <wad@...omium.org>, Mark Brown
	<broonie@...nel.org>, Muhammad Usama Anjum <usama.anjum@...labora.com>,
	<linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional
 into test metadata

On Thu, Jun 12, 2025 at 12:42:42PM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 12, 2025 at 05:23:01PM +0200, Thomas Weißschuh wrote:
> > On Thu, Jun 12, 2025 at 11:58:01AM -0300, Jason Gunthorpe wrote:
> > > On Thu, Jun 12, 2025 at 04:27:41PM +0200, Thomas Weißschuh wrote:
> > > 
> > > > If the assumption is that this is most likely a kernel bug,
> > > > shouldn't it be fixed properly rather than worked around?
> > > > After all the job of a selftest is to detect bugs to be fixed.
> > > 
> > > I investigated the history for a bit and it seems likely we cannot
> > > change the kernel here. Call it an undocumented "feature".
> > 
> > I looked a bit and it seems to be mentioned in mmap(2):
> > 
> > 	For mmap(), offset must be a multiple of the underlying huge page size.
> > 	The system automatically aligns length to be a multiple of the underlying huge page size.
> 
> Oh there you go then :) Horrible design. No way for userspace to know
> what the rounded up length actually was and thus no way for
> userspace to unmap it.

OK. I think we would have to skip those cases then.

> > > MAP_HUGETLBFS rounds up the length to some value, userspace has to
> > > figure that out and not pass incorrect lengths.  The selftest is doing
> > > that wrong.
> > 
> > The selftest would be more robust if MAP_FIXED is replaced by
> > MAP_FIXED_NOREPLACE. Even with the new explicit skip logic it should
> > make debugging easier if something goes wrong.
> 
> The point is to replace something that is already mapped there, though
> I no longer remember why it is working like this.

By replacing MAP_FIXED with MAP_FIXED_NOREPLACE, at the existing
two places, the selftest crashed at early setup_sizes...:

iommufd: iommufd.c:53: setup_sizes: Assertion `vrc == buffer' failed.
/nicolinc/iommufd_selftest.sh: line 19: 21487 Aborted
		(core dumped) tools/testing/selftests/iommu/iommufd
strace:
mmap(0xffff80000000, 1048576, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS|MAP_FIXED_NOREPLACE, -1, 0) = -1 EEXIST (File exists)

This one doesn't MAP_HUGETLBFS btw...

Thanks
Nicolin